Acoustic filter



July 4, 1961 w 5 EVERETT 2,990,907

ACOUSTIC FILTER Filed June 11, 1959 3 Sheets-Sheet 1 ATTORNEY.

W. S. EVERETT ACOUSTIC FILTER July 4, 1961 5 Sheets-Sheet 2 Filed June 11, 1959 INVENTOR ILHELM EyE/aETT Y fl?" TORNE'X'I y 4, 1961 w. s. EVERETT 2,990,907

ACOUSTIC FILTER Filed June 11, 1959 5 Sheets-Sheet 3 7 9| 2 a 4 567851 2 2 3 4 56 I300 I 00 20000 FREQUENCY m CYCLES PER SECOND NO 0 o o o o o 0 N w M I l 1 I I I czgaosev Asazwzl Dusnoau =40 51391320 INVENTOIE W/LHEL/V 5. E VEEETT 141 TTORNEY,

United States Patent 2,990,907 ACOUSTIC FILTER Wilhelm S. Everett, 1349 Main St., Santa Paula, Calif. Filed June 11, 1959, Ser. No. 819,779 6 Claims. (Cl. 181-54) This invention relates to an acoustic filter to be employed in connection with a stream of compressible fluid, to limit or decrease the fraction of the energy of the stream present in frequencies in the audible range.

This application is a continuation in pant of applicant's application Serial No. 526,725, filed August 5, 1955, which is now abandoned.

As is well known, streams of compressible fluids, such as gases flowing in confined channels, such as pipe systems, upon intake from and discharge into the atmosphere generate a great deal of noise depending on their velocity and physical characteristics and on the physical characteristics of the engines, pumps or compressors in the system. Additionally, the acoustic vibrations tend to generate mechanical vibration in the system with consequent damage which frequently may be quite severe. The frequency and wave shape of the energy input into the flowing system also modifies the nature of the acoustic energy in the fluid stream and the nature of its acoustic spectrum.

The acoustic problem of noise and energy loss due to noise in such systems where this problem has been found most aggravated has resulted in the application of various muffler devices, which in principle depend on absorbing the sound by reducing the pressure peaks, i.e., the amplitude of the acoustic wave energy in the flowing stream without substantial change in the acoustic spectrum. It is conventional in acoustic theory to consider acoustic systems in terms analogous to electrical circuits. Viewed in this manner these devices in various forms introduce a resistance in the circuit to cause a damping of the wave energy by reducing the amplitude of the waves. This resistance may be a series resistance as when baffiing of various sorts in introduced into the flowing stream, or a parallel resistance as where the pressure pulses are permitted to escape in a direction perpendicular to the flowing stream into a baffling chamber filled with sound absorbing material and pressure is reduced by the flow of the stream in and out of the baffling chamber.

Studies which I have made of the energy distribution in the acoustic spectrum of such flowing fluid streams show that these streams resonate in certain frequency ranges.

In all such previously mentioned prior art and other similar systems, the sound absorption is not selective, since operating merely to introduce a resistance, the nature of the acoustic frequency distribution in the acoustic spectrum is substantially unaltered but the energy in the various wave lengths are reduced by reason of the energy loss in overcoming this resistance. In order to reduce the pressure peaks of the acoustic waves in the region of resonance, considerable resistance is required.

The acoustic energy loss thus results in a large pressure drop in the system, introducing a substantial back pres sure and a drop in the efliciency of the system. Additionally, frequently these systems introduce reflective surfaces, which because of interference phenomena generate standing waves and thus also generate localized areas of high pressure which add to the back pressure in the system.

Instead of relying solely or in the main on the sound absorbing effect of acoustic resistance elements, I so modify the acoustic characteristics of the pipe line at the point where the acoustic energy is to be modified so as to selectively absorb the acoustic energy from the entire acoustic spectrum without introducing any substantial back pressure into the system.

I have devised an acoustic mute which absorbs selected frequencies of the acoustic spectrum, which may be at the resonance frequencies of the acoustic system into which it is placed. Also, especially Where the acoustic spectrum contains a material proportion of the acoustic energy in relatively low frequencies and/or relatively high frequencies, I may cut off such high frequency or low frequency wave or both low frequency and high frequency waves. By so doing I may obtain a more uniform distribution of acoustic energy over the whole acoustic spectrum. I accomplish these results by introducing into the system acoustic resonators whose frequency response is in the frequency region in which noise suppression is desired.

These acoustic resonators have a capacitance and inductance, using the language of the electrical analogies employed in acoustic theory, so that their natural frequencies are such that they will absorb the energy of the acoustic waves at the desired location in the acoustic spectrum and in the region of acoustic repsonse of the resonators, and do not introduce any substantial resistance to the flow of the fluid whose noise is to be suppressed.

To accomplish these results I place in the flow path of the fluid, an acoustic resonator in the form of a closed chamber, having an entrance port or ports so positioned with respect to the direction of flow of the fluid so that the acoustic wave pressure may be communicated from the flowing stream directly through the port. The chamber being full of the fluid is thus subject to compression and rarefaction induced by the propagation of the wave energy through the port. The gas in the chamber acts like a compressible spring of frequency response determined by the free volume of the chamber and the area and geometry of the port entrance.

The filter thus may act to reduce the magnitude of the positive and negative peak frequencies of acoustic waves within the range of frequency to which the filter responds without introducing any substantial back pressure into the system. Since the noise level is determined by the magnitude of the resonance peaks, the reduction in the energy in these peaks causes a reduction in the noise level of the stream of fluid without requiring reduction in the amplitude of the waves across the entire acoustic spectrum as is the case in prior art damping known to applicant. Such filters are particularly useful when it is desired to selectively absorb relatively low frequencies.

Where I desire to add to the action of the low frequency response acoustic filter, one having a higher frequency absorption of the acoustic spectrum, I use as part of the aforesaid filter or as a separate element an acoustic filter having a relatively higher frequency response. In order to obtain the proper frequency response characteristics of the acoustic filter at the selected high frequencies, the ratio of the port area to the volume of the acoustic filter is preferably made greater than for the filter of lower frequency response. In order to reduce the volume of the filter I introduce a filter element so as to divide the chamber into a plurality of small conduits which communicate with the entrance ports, and if desired, also with each other. In this way I reduce the free volume of the acoustic filter chamber to obtain the proper ratio of the port area to the free volume in communication therewith.

I also place the port area so that the acoustic wave pressure may be communicated to and through the ports into the free volume of the filled acoustic filter. In a pretferred embodiment, the flow of the fluid is made to pass over each port, for example, in a direction parallel to the plane of the port entrance. The characteristics of this Patented July 4, 1961 acoustic filter are that it absorbs high frequency acoustic energy and has a high frequency cut-off point.

When the relatively low frequency filters are used in conjunction with the high frequency filter described above, I may attenuate and suppress the low and medium frequencies of the flowing fluid and the high frequency acoustic waves, and thus obtain a large reduction in the noise level of the flowing stream without introducing any substantial back pressure into the system.

The general form of the low frequency filter is a resonance chamber having a port entrance to which is connected a conduit of low frictional resistance and lowrefiectance in the form of a horn or a difiuser with a take-01f discharge outlet in a direction to one side of the port opening of the resonance chamber. In the preferred embodiment the flow is discharged generally perpendicular to the direction of flow and to the axis of the port opening to the resonance chamber.

In the general form of the relatively higher frequency filter the flow of gases is again directed through the diffuser and horn and the flow is similarly directed through a passageway in a direction to one side of the axis. The passageway is formed of a foraminated boundary wall which forms also the'wall of a chamber filled with mate-' rial to reduce the free volume of the chamber, as will be further described. In a preferred embodiment the passageway is made to surround the diffuser as, for example, in the form of an annulus, so that the surface area of the boundary walls may be great in relation to the volume to permit a suflicient number of small holes to be placed so that the ratio of the total ports area, i.c., the holes, bear the desired relation to the volume of the chambers to give desired resonance characteristics.

The passageway and chamber are adjusted inwidth and thickness along the flow path of the fluid so that the resonance characteristics of the chamber may be spread over the desired band of frequencies.

I may instead of using the filled chamber tuned to the higher frequencies in combination with the free volume filter tuned to the lower frequencies, use it alone when the acoustic spectrum of the fluid has so little energy. in the lower frequencies as not to be disturbing.

Instead of the diffuser acting as the intake and the side take-off as the outlet, the flow may be reversed.

The invention will be more readily understood from the following description of some preferred embodiments thereof, taken in connection with the appended drawings wherein:

FIGURE 1 is a view in elevation, with the parts shown in section, of one form of the invention device;

FIG. 2 is a section taken on line 2- -2 of FIG. 1;

FIG. 3 is a view in elevation, with the parts shown in section, of another form of my device;

FIG. 4 is a section taken on line 44 of FIG. 3;

FIG. 5 is a section taken on line 55 of FIG. 3;

FIG. 6 is an enlarged fragmentary section of a modified detail of the device of FIG. 1 taken in the direction of the arrows 66 in FIG. 1;

I FIG. 7 is a fragmentary section taken on line 7-7 of FIG. 6;

FIG. 8 shows the device of FIG. 1 employed on an exhaust mufier on an internal combustion engine; and

FIG. 9 is a graphic illustration of the attenuation characteristics of certain acoustic filters according to the invention.

Referring to FIGS. 1 and 2, there is shown an acoustic filter or mute 10 having an elongated conical entry tube 12, preferably formed of sheet metal, having a fluid entrance port 14 and a flange 18 adjacent said port, said flange being adapted to be connected by means of bolts 20' to the exhaust of a unit, e.g., an internal combustion engine, to attenuate the undesirable frequencies of the acoustic energy of the fluid discharged from said unit.

The enlarged flared end 22 of the conical tube 12 has connected to the outer periphery thereof an annular pancake type construction generally represented by the numeral 24, also preferably formed of sheet metal. Said pancake is composed of an annular chamber 26 having a rounded or bumped annular wait 28, the inner periphery of which is welded or otherwise suitably connected at 30 to the wall of tube 12. The outer periphery of the bumped wall 28 is bent outwardly for a short distance forming a Wall 32 parallel to the axis of tube 12, and said wall 32. is then bent inwardly normal to the axis of tube 12 to form an annular wall 34, which is connected to the wall of tube 12 e.g., by welding, as at 36. Hence, it is seen that the annular chamber 26 is bounded by walls 28, 32, 34, and the outer wall portion 38 of tube 12. The chamber wall 28 is rounded or bumped for stifiness and to increase the natural frequency so as to prevent vibration in the throat 22 at low frequencies.

The other element of the pancake construction 24 comprises an annular chamber 40 which is spaced from annular chamber 26 in a direction axially along tube 12. The annular chamber 40 is formed at the inner end of a cylindrical resonance chamber 42 which is bumped outwardly at its end 44 to provide stiffness. The resonance chamber 42 is mounted on the axis of the conical tube 12 and has a diameter about equal to the diameter of the outer wall 32 of chamber 26. Along the inside wall of chamber 42 near the inner end thereof is an annular ring 46, e.g'., formed of sheet metal and connected as by Welding at 48 to said wall. Ring 46 extends inwardly of chamber 42 in a plane normal to the axis of tube 12 to a point axially oppositethe flared end 22 of tube l2. Ring 46 is bent outwardly a short distance to form a short cylindrical wall 50 whose axis coincides with the axis of tube 12, said wall 50 having a diameter substantially equal to or greater than the diameter of the flared end of tube 12. The opposite end of wall 56 is bent outwardly to form an annular wall 52 which extends toward wall 34 of chamber 26, at an acute angle to said wall 34, wall 52 being connected to the inner end of the cylindrical wall of resonance chamber 42. Chamber 40 is thus bounded by ring '46, walls 50 and 52, and the inner wall portion 54 of the resonance chamber 42.

The resonance chamber 42 with the chamber 40 formed at the inner end thereof is connected to the flared end 22 of tube 12 by means of a plurality of lugs or arms 58 connected by suitable means, such as welding, at spaced intervals about the outer periphery of walls 34 and 52 of chambers 26 and 40. In this manner the adjacent walls 34 and 52 are spaced from each other forming an annular passage 6i) between chambers 26 and 40, said passage communicating, via the large intermediate passageway 61 between tube 12 and resonance chamber 42, with the open flared end 22 of tube 12 and the circular open end 62 of the resonance chamber 42, formed by the annular wall 50 thereof. The annular passage decreases in depth or thickness from its inner end 64 to its outer end 66 which communicates with the atmosphere.

A series of concentric rows of round spaced holes or apertures 70 is formed in the annular wall 34 of chamber 26, and a series of concentric rows of similar holes 72 is formed in the adjacent annular wall 52 of chamber 40. These holes form entrance ports for the fluid whose acoustic energy is to be selectively attenuated according to the invention. The size of such holes or apertures is determined in accordance with certain design parameters of the acoustic filter, as discussed more fully below. In chambers 26 and 40 is loosely packed a porous filler material 74 for reducing the free volume of said chambers, and which may be a mineral fiber or wool such as fiber glass, asbestos, or similar material, or may be in the form of a metallic wool or fibers such as steel wool. The filler material need not be a sound-absorbing material, so long as it is of a foraminons or porous nature and has a plurality of small passages or interstices of more or less uniform size distributed therethrough and which may be in communication with each other. When loosely packed into chambers 26 and 40, the small passages and interstices of the filler material are also in communication with the entrance ports or holes 70 and 72.

While the use of fiber glass 74 as a filler in chambers 26 and 40 constitutes a preferred embodiment, I do not rely on the use of this material or the other filler materials noted above for their sound-absorbing properties, and hence filler materials of low' sound-absorbing properties can be used. Thus, for example, a series of corrugated sheets 76 (see FIGS. 6 and 7) can be connected across walls 28 and 34 of chamber 26 and across Walls 46 and 52 of chamber 40, the sheets being juxtaposed adjacent each other to form a large number of small passages 77 in communication with holes 70 and 72. These corrugated sheets do not function essentially as a sound-absorbing material, but rather to reduce the free volume of chambers 26 and 40, and form the minute Passages or conduits 77, whereby the acoustic characteristics of the fluid are modified to selectively absorb the acoustic energy of the desired portion of the frequency spectrum.

For example, the device of FIG. 1 can be designed and employed, where the fluid has an acoustic spectrum with a resonance peak at about 50 c.p.s., to attenuate the low frequencies of the acoustic spectrum of a fluid, e.g., frequencies between 20 and 150 cycles per second (c.p.s.), with a maximum response at 50 c.p.s., the fundamental disturbing frequency of the particular fluid. The fluid whose acoustic energy is to be attenuated passes into the entrance port 14 of the conical tube 12, and passes axially through the tube, the passage 61 adjacent the flared mouth 22 of the tube, and into the closed resonance chamber 42, the pressure of the fluid decreasing as it passes along conical tube 12 to the flared end 22 thereof. Since resonance chamber 42 is full of fluid, the chamber is subjected to compressions and rarefications caused by the propagation of wave energy.

The conical tube 12 need not be a horn, and thus may be designed to have no low frequency cut-off point. It thus may act entirely as a diffuser to drop the entry or exhaust pressure. It is noted that the diffuser cone 12 has a shape such that there are no reflective surfaces. The percentage of the energy reflected from the diffuser surface may, in fact, be held down to 15% or less, and essentially no back pressure is developed. The conical entry tube, may, however, be designed as a horn to attenuate acoustic wave energy of low frequencies at or below the resonance frequency of the resonance chamber 42, where the low frequency energy to be attenuated is of wave lengths to permit horns of reasonable dimensions to be employed, for example, where acoustic energy frequencies below about 300 c.p.s. are desired to be attenuated. It is noted that there is no restriction between the discharge end 22 of the diffuser cone 1'2 and the entrance port 62 of the resonator chamber, said end 22 and entrance port 62 being of substantially the same area and in alignment with each other. The entrance port 62 to the resonator is normal to the axis of fluid flow through the conical tube 12, resulting in little or no energy loss due to resistance to flow into and out of the resonator. Thus, all of the energy of the fluid is recovered except that amount which is used up in the entropy of the compression-rarefication cycle of the acoustic filter. This amount of energy loss is an insignificant fraction of the total energy.

The resonator 42 is preferably tuned as a band rejection filter which may have a fairly sharp absorption peak at about the fundamental disturbing frequency of the fluid passing through the filter. Thus, the resonator is, for example, designed to have a resonance point at the frequency resonance point of the fluid stream or at the frequency at which maximum attenuation is desired, e.g., in the above example 50 c.p.s., by properly proportioning the area of the port entrance 62 of the resonator to the free volume of the resonator. The resonator-will respond to a frequency band which is maximized at the theoretical response frequency, as shown more clearly hereinafter.

The fluid and acoustic wave energy then pass from the passageway 61 into the inner circular entrance 64 of the annular passage 60, progressing outwardly therethrough in a radial direction between the adjacent walls 34 and 52 of the pancake 24, to the circular exit port 66 of passage 60. Hence, it is seen that the takeoff of the fluid flow from the first filter element comprising the resonator chamber 42 to the second filter element in the form of the pancake construction 24 comprising the annular chambers 26 and 40, is located at an angle to the original direction of fluid flow. The surfaces of the second filter element can be made either parallel to, or at such an angle to the original direction of fluid flow so that no substantial degree of wave reflection and thus no substantial degree of interference phenomena are encountered. It is seen in FIG. 1 that the filter surface 34 of the pancake 24 is positioned at about right angles to the original flow of fluid while the filter surface 52 of the pancake is at an acute angle to such flow.

It is to be observed that along the path of flow of the fluid, at each point, the circumference at the annulus 34 is many times greater than the linear dimension of each of the chambers 26 and 40 in a direction transverse to the circumference.

The flow of fluid through annular passage 60 over ports 70 and 72 in the passage Walls 34 and 52 is in a direction substantially parallel to the plane of the port entrances or to such walls. The acoustic pressure waves generated by the fluid passing through passage 60 travel through the ports 70 and 72 into the interstices or conduits, e.g., 76, formed by the filler material in the annular chambers 26 and 40, and the fluid in said conduits is subjected to compressions and rarefications, causing attenuation of the acoustic energy in the frequency range for which the second filter element comprising the pancake construction 24 is designed. The frequency response of said second filter element is at higher frequencies the greater is the ratio of the areas of the port entrances 70 and 72, to the free volume of the chambers 26 and 40, and also to the annular area of passageway 60 in a direction transverse to the annular chambers 26 and 40. It is thus seen that by filling the chambers with a loose porous material, such as fiber glass, so as to reduce the free volume of said chambers, for a given port area the ratio is increased, and the frequency response of the second filter element or pancake 24 can be made substantially higher than the frequency response of the first filter element comprising the resonator 42, since the area of the port entrance 62 to the unfilled free volume of the resonator chamber 42 is substantially less than the ratio of the total port areas 70 and 72 to the free volume of chambers 26 and 40.

Hence, it is seen that the acoustic filter 10 comprises a low frequency response free volume resonator chamber in series with a high frequency response filled volume annular pancake element, these elements having a beneficial combined effect as illustrated by curve A in the plot shown in FIG. 9. Curve A represents the acoustic attenuation or absorption characteristics of a particular filter constructed as illustrated in FIG. 1. The filter was designed to produce maximum noise suppression at c.p.s., the greatest noise producing frequency of the particular fluid involved, with intermediate noise suppression of relatively constant value over a relatively broad frequency band ranging from about 200 c.p.s. to about 700 c.p.s. and a cut-off at about 7000 c.p.s.

Illustrating the principles of my invention, and referring to FIG. 9, by properly proportioning the ratio of the port area 62 to the volume of resonator chamber 42, the device may be built so as to produce maximum frequency response of the resonator at 50 c.p.s., and it is seen that the filter 10 will absorb a maximum of decibels of acoustic energy at this peak noise disturbing frequency with an attenuation of. between 25.5 andv 60 decibelsat frecglencies between 20 and 50V c.p.s., and about 25.5. to 60 decibels between 200 and 50 c.p.s. Hence, the noise level at the peak disturbing frequency of 50' c.p.s. is reduced 60 decibels from the original noise level.

While the resonator element 42 of the acoustic filter 10 attenuates the acoustic energy at low frequencies, the pancake element 24 of the filter is designed to cooperate with the resonator to attenuate the acoustic energy at high. frequencies, of the same fluid previously treated in the resonator. For example, if the peak disturbing. frequency at high frequency levels of the fluid is 400: c.p.s., and it isdesired to obtain maximum attenuation at this frequency in the pancake element 24, employing the equation Am W where A is thetotal port area, V is the free volume, f is the frequency andK is a constant, the total volume of chambers 26 and 40 would have to be or- A of. the volume of the resonator 42 for equal port areas; or for equal volumes of chambers 26 and 40, and resonator 42, the port areas 70 and 72 of chambers 26 and 40 would have to be 4096 times that of the port area 62. of the resonator. To meet this, the sum of the port areas formed by apertures 70 and 72 are made as large. as is practical. by employing the. pancake construction and the freevolume of. chambers 26 and 40 is substantially reduced by loosely packing said chambers with the fillermaterial described above.

Basedonthe aforementioned criteria, it has been found that for most applications the total open area represented by the apertures 70 in wall 34 or apertures 72 in' wall 52 will notexceed' 20% of the overall area of the wall surface. In fact, in most applications the open area as represented; by the apertures in the particular wall will represent considerably less than 20% of the overall area of the wall. For example, on lower frequency applications from 200 to 400 cycles per second, the percentage figure is approximately 5%. plications, for example in the case of gas jet silencing, where frequencies approach 9600 cycles per second is it necessary or feasible to have the percentage of open area exceed 20%.

It is essential, however, that the open area, that is the I spacing. and size of the apertures 70 and 72, be preselected according to the resonant frequency to be absorbed. Thus with the foregoing criteria in mind, with higher frequency noises, more apertures are used in order to achieve a smaller effective volume behind each aperture. On the other hand, fewer apertures are employed With lower frequencies to obtain a larger effective volume behind each aperture.

While the above criteria enable the pancake element 24 of the filter to absorb in the region of resonance at 400 c.p.s., in the illustrative example used in connection with FIG. 9, plot A, in order to broaden the frequency band of frequencies which are absorbed so that the-filter will absorb not only in the region of resonance but" also on both sides thereof, i.e., so that it will also absorb frequencies higher than resonance, or frequencies lower than resonance in substantial proportion, having chosenthe volume and port areas, the thickness of the chambers 26 and 40, at any cross section of the pancake 24 may be adjusted according to the following principles.

Theabsorption characteristics of the chambers 26 and 40Idepend on the diameter of the ports and. number of ports, i.e;, their spacingthe height of the annular passage and. the linear dimension. of the chambers 26 and'40 in Only in exceptional ap- Cir a direction perpendicular to. the plates or chamber. walls 34 and 52.

, Thus, for a sharply tuned resonator, the plates should be made parallel and the thickness of the chambers 26 and 40 uniform along the entire radius R of the annular passageway 60. The resonance frequency f of the'chamher at any cross section perpendicular to the radius, such as represented by thedotted line X in FIG. 1, is proportional to K /T/H where K is a constant depending on the velocity of sound and the area and spacing of the ports, T is the average value of the thickness of the chambers 26 and 40 measured at any point along the radius, and H is the average height or thickness parallel to the axis of the annular passage measured at such'point and at the passage inlet 64. Where the values of T and H are maintained constant at each cross section along the radius, the frequency to which the chamber is tuned at each cross section will be the same and will be directly proportional to the ratio /d/p where a is the diameter of the hole and p is the distance between the holes.

If, however, Tor H, or both T and H are varied along the radius of the annular passageway 60, then the frequency response of the elemental sections of the annular passage taken transverse-thereto, for example, at the section located at the place marked X in FIG. 1, without changing the diameter or spacing of holes and 72, will vary, becoming greater in the direction of flow as the value of /T/H increases and decreasing in the direction of flow as the value of /T/H becomes smaller. Thus, the frequency band may be broadened to the side of the higher frequencies by increasing the ratio of the /T/H as for constant values of d and p or by increasing the. ratio of Val/p for constant values of /T/H as will be understood by those skilled in the art. In the form illustrated in the figure I prefer to vary the value of /T/H rather than vary the ratio of d/ p, since this ratio Vii/p is determined in part by the requirement that the values of d and. p be such as to retain. the packing in place and particularly since a. and p are de pendent variables, whereas T and H are independently variable.

In the form as'illustrated in FIG. 1 the value of. /T/H. increases in the direction of flow from the entrance to the exit of the annular passageway and the resonant frequency is thus a broad band of frequencies with the lowest frequency corresponding to the geometry at the entrance to the annular chamber.

Thus, as seen in curve A of FIG. 9, the acoustic energy is attenuated by the pancake structure 24 over an approximate frequency band ranging from about 200 to about 700 c.p.s., with an acoustic absorption of about 25.5 decibels over this range. From about 700 to about 7000 c.p.s. the attenuation decreases until at 7000 c.p.s. and higher there is no longer any attenuation of acoustic energy, so that 7000 c-.p.s. is the approximate so-called cut-off of the acoustic filter 10. If the pancake element 24 of the filter were omitted, the noise attenuation would decrease rapidly above about 100 c.p.s., with a cut-off point at a much lower frequency than when the pancake element is used. Hence, this shows the marked advantage of employing the pancake element 24 in conjunction with the resonance chamber 42 where the acoustic spectrum has considerable energy in the frequencies above the low resonance peaks. This is readily apparent from FIG. 9, in that attenuation of both low and high noise producing frequencies can be attained, with a broadwing of attenuation at the high frequencies, but with little or no absorption of energy in the frequencies above the audible range, i.e., 15,000 to 20,000 cycles or more.

One application of the acoustic filter or mute 10 described above is in an internal combustion engine, as illustrated in FIG. 8. Numeral represents the engine, with:the short pipes 82 representing the exhaust outlets 9 from the individual cylinders to the exhaust manifold 84. The acoustic filter is connected to the end of manifold 84, with the small end 14 of the conical tube 12 of the mute connected into the manifold.

As an illustrative example of results which have been obtained, when operating the above engine with a conventional mufller, 7 /2 inches of water pressure were generated at point 86 just ahead of the prior art muffler and 9 /2 inches of water manifold pressure at the opposite end 88 of the exhaust manifold, making a 2-inch drop in pressure.

With the mute 10 of the design described above, the pressures were 1% inches of water at the discharge end 86 of the manifold ahead of the mute and 2 /2 inches at the opposite end 88 of the exhaust manifold. The drop in back pressure in the manifold was equal to an increase of in the volume of scavenging air in the cylinders and a drop of 17 in the exhaust temperature, equivalent to 10% increase in horsepower output of the engine. The temperature at the exhaust 82 from each cylinder into the manifold was more uniform in the case of the invention mute 10 than for the conventional muffler.

While I do not wish to be bound to my theory of the reasons for the above advantages of operation employing the invention mute 10, I believe that in the case of the mufflers of the prior art, the baffling therein introduces frictional resistance to flow and also introduces reflective surfaces which generate wave interferences and thus standing waves in the discharge conduit, which act to throttle the flow of the exhaust gases and create back pressure. In the structure of my invention these are largely eliminated since I rely not on the damping resulting from friction or resistance to flow, but on the acoustic resonance properties of the acoustic filter or mute of the invention, which does not rely on the introduction of friction and resistance to flow. With a conventional mufller the cylinders of engine 80, discharging against the standing wave produced in the manifold 84, would also have a higher temperature than those cylinders of the engine not discharging against such standing wave, resulting in non-uniform cylinder temperatures. Since my mute acts to decrease the exhaust back pressures against the engine cylinders, it also improves the operation of the engine.

Assuming a wave length of 20 feet in the embodiment of FIG. 8 employing a conventional muffler, any reflections at 10 feet or any odd multiples thereof create standing waves in the manifold 84, e.g., at the dotted line position 89, depending on the characteristics of the pipe, and the cylinder discharging into that area sees a high pressure area and does not scavenge properly, thus reducing efliciency. Such standing waves are not produced employing my acoustic filter. In my filter 10 the pancake element interposes minimum surface transverse to the direction of propagation of the acoustic wave, thus preventing the system from creating standing waves.

It is to be emphasized that in order to properly build up a resonant frequency, the opposing walls 52 and 34 of the pancake sections or chambers 26 and must be formed of relatively rigid matenial, preferably sheet metal. Such a construction, as previously mentioned, contrasts the prior art in which fabric walls, for example, are employed to absorb sound by fiow resistance rather than by resonating or reactive surfaces. Thus, construction of the present invention is directed towards creating an acoustic filter with minimum fluttering, paneling, and/or vibration of the sidewalls defining the fiow path. As a consequence, of such a stiff or relatively rigid structure, the pressure drop and flow resistance through the unit may be held to a minimum.

Referring now to FIGS. 3, 4 and 5, there is shown a modification of the acoustic filter 10 of FIG. 1, and wherein the low frequency free volume resonator 42 is eliminated. Thus, the filter of the instant modification is applicable where the resonance frequencies are in the higher range of the acoustic spectrum. In this modifica tion the acoustic filter comprises a conical tube 92 similar to tube 12 of FIG. 1, except that it is of shorter length. Tube 92 has at its reduced end a fluid port 94 and a flange 96 adjacent said port, said flange being adapted to be connected by means of bolts 98 to the inlet of a unit such as a compressor (not shown) to attenuate the undesirable frequencies at the noise peaks of the fluid entering said unit.

The enlarged flared open throat 100 at the opposite end of conical tube 92 is connected to a pancake construction 102 similar to pancake 24, except as to the outer pancake element 104. Pancake 102 is composed of an annular chamber 106 having a rounded or bumped annular wall 108, the inner periphery of Whichis suitably connected as by welding to the outer periphery of the wall of tube 92. The outer periphery of bumped! wall 108 is bent to form a short outer wall 110, wall 110- then being bent inwardly normal to the axis of tube 92 to form an annular wall 112, which is connected to the flared end of the conical tube, e.g., as by welding at 114.. Thus, annular chamber 106 is bounded by walls 108, 110,. 112 and the Wall portion 116 of tube 92.

The other element 104 of the pancake 102 is a cylindrical chamber 118 which is spaced from annular cham ber 106 in a direction axially along tube 92. Chamber 118 is formed of an inner conical Wall 120, the apex 122". of which is on the axis of tube 92. At the outer periph cry of conical wall is connected a dome shaped head. 124. The chamber 118 has a diameter about equal to the diameter of the outer Wall 110 of chamber 106. The element 104 forming the cylindrical chamber 118 is con-- nected to the flared end 100 of tube 92 by means of a series of arms or lugs 128 connected as by welding at. spaced intervals about the outer wall 110 of chamber 106, the opposite ends of said lugs being connected to the outer wall 129 of chamber 118, to thereby connect said walls and maintain them in spaced apart relation. Thus, the adjacent walls 112 and 120 are also spaced from each other forming an annular passage 130 between chambers 106 and 118, communicating with the open flared end 100 of conical tube 92. The annular passage 130 decreases in depth from the inner end 132 thereof to its outer end 134, which communicates with the atmosphere.

A series of concentric rows of round spaced holes 136 is formed in the annular wall 112 of chamber 106 and a series of concentric rows of similar holes 138 is formed in the adjacent conical wall 120 of chamber 118, the size of said holes being made in accordance with the design criteria previously explained. In chambers 106 and 118 is placed a filler material of the type described above, e.g., fiber glass, and represented by numeral 140, to form numerous small conduits in communication with the ports or holes 136 and 138, and to substantially decrease the free volume of chambers 106 and 118. In this manner the ratio of the total area of ports 136 and 138 to the total free volume of chambers 106 and 118 is increased in accordance with the invention principles as previously explained.

It will be observed that the annular passageway flares in the direction of flow so that the value of /T/H increases in the direction of flow, T remaining substantially constant. Thus, the resonant frequency of the chamber is in a broad band with the lower frequency determined by the geometry of the intake entrance to the annular chamber. For example, these characteristics are illustrated by curve B in the plot of FIG. 9, which curve represents the operational characteristics of filter 90. by curve B is designed to have a resonance frequency over a broad range maximizing in the range of about 300 to 800 c.p.s. with strong absorption in the range of 20 to 1500 cycles, falling off above 1500. Above 1500 cycles, absorption takes place in smaller degree It is noted that the acoustic filter 90 illustratedthroughout the audible range up to 20,000 cycles, with a practical cut-off just above the audible range. This filter unit would accordingly act efiiciently on fluid generating an acoustic spectrum resonant in the range of 300 to 800 cycles and having considerable energy in. the frequencies above and below such resonant frequencies. Passage of the fluid, e.g. by suction, through the annular duct 130 of the pancake between the ad jacent walls 112 and 120 of chambers 106 and 118, causes the Wave energy of the fluid to be propagated through the ports 136 and 138 of the pancake into the small conduits formed in the fiber glass or other filler material 140 to attenuate the acoustic energy of the fluid at the peak frequencies of noise level noted above and for which the filter 90 is designed. The fluid then passes through conical tube 92 and out the exit port 94 thereof, tube 92 again functioning essentially as a diffuser.

As illustrated in FIG. 9, curve B, filter 90 is designed for maximum attenuation at 300800 c.p.s. with the frequency band for maximum response of the filter broadened to give an attenuation of 35 to 36 decibels and an attenuation varying down to about 27 decibels in the lower frequencies and down to about decibels at 10,000 cycles. It is thus seen that the pancake construction 102 of filter 90, with the concentric rows of ports 136 and 13S communicating with the annular passage 130 and with the interiors of chambers 106 and 118, which have their free volume reduced by the porous filler material 140, results in a broadening of the frequency range at which maximum absorption takes place. Because of the increasing width of duct 130 in the direction of fluid flow from the fluid entrance 134 of the duct at the outer periphery of the pancake structure 102 to the inner duct exit 132 adjacent the flared end 100 of the conical tube 92, the range of maximum absorption is broadened mostly at frequencies above the resonance frequency of 400 c.p.s., i.e., from 300 to 800 c.p.s.

Again, as heretofore mentioned in conjunction with FIG. 1, it is essential to the present invention that the apertures 136 and 138 be of pre-selected size and spacing according to the resonant frequency to be absorbed and in accordance with the criteria heretofore explained. Also, except for applications of unusually high frequency, the percentage open area of the walls 112 and 120, as represented by the ports 136 and 138, respectively, will not exceed Also, of course, it is necessary as heretofore mentioned that the walls 112 and 120' be formed of relatively rigid, stiif material in order to properly build up resonant frequencies. In consequence, it is preferred to form these walls, as well as the entire unit of FIG. 3, of metal.

While in the filter structures shown in FIGS. 1 and 3, the radial passages 60 and 130 become narrower from the inner to the outer periphery thereof, such passages can be made narrower in the opposite direction, i.e., they may become narrower proceeding from the outer to the inner periphery of such annular passages. This does not change the function of the pancake, but tends to broaden the range of frequency response thereof in a direction above or below the resonance response frequency at the entrance to the passages, to thereby broaden the frequency range within which disturbing frequencies of the acoustic spectrum are absorbed.

The above specific examples and values were given merely to illustrate the principles of my invention and are not to be taken as limiting my invention. Those skilled in this art will known how to apply the above principles to give the desired degree of attenuation at various frequencies and to obtain the desired position of the resonance frequencies of the filters in the acoustic spectrum and the width of the band of frequencies within the resonance range of the filters.

In FIG. 3, it is noted that chamber wall 120 has been shown as conical. However, such wall may have other geometric configurations, and may, for example, be coni2 cave looking fromthe flared end of tube 92, or wall may be parabolic, or even in the form of a plane parallel to the adjacent wall 112 of chamber 106.

Further, it is noted that element 104' of the pancake structure 102 is in the form of a cylindrical chamber rather than an annular chamber, such as 106. The central portion 150 of cylindrical chamber 118, located opposite the flared throat 100 of tube 92., functions to a degree as a resonator chamber which is filled with a loose filler material. Also, the central portion 152 of conical wall 120, located opposite the flared throat 100 of the conical tube 92, serves to reflect acoustic waves striking said central wall portion 152 back into the conical diffuser tube 92, and aids in dissipation of such acoustic wave energy by multiple reflections.

While I have described the small holes or ports 70, 72 and 136, 138 as being formed in concentric rows in the adjacent chamber walls, these apertures need not be in concentric rows, and may, for example, form squares with the holes approximately equally spaced from each other.

I may cover the outer peripheral annular opening of the annular passage 60 or with a screen, if desired, to permit flow of fluid therethrough, while preventing the ingress of foreign materials, such as bits of paper and the like from outside the unit, into said annular passages. The use of such a screen does not affect the functioning of my acoustic filter in the manner previously described.

While the pancake elements of the pancake structure 24 or 102 have been shown to be round or circular, these pancake elements can be square or polygonal in outer peripheral form. The term annular chamber and annular passage as employed in the specification and claims is intended to denote a ring-shaped chamber or passage, and said ring may be of circular, square, or polygonal shape, so long as it is continuous.

From the foregoing, it is seen that I have designed an acoustic filter or mute which absorbs selected frequencies of the acoustic spectrum, which may be low or high frequencies, or both low and high frequencies. The filter can be designed for a particular cut-ofi" frequency, that is, a frequency below which or above which, the filter no longer absorbs or attenuates the acoustic energy in substantial amounts. These advantages are obtained without introducing any substantial amount of resistance to the flow of fiuid whose undesirable noise peaks are being attenuated by my device, and hence without development of any substantial amount of back pressure.

The principles of my invention as embodied in my mute or filter are applicable to any unit in which a cylinder and piston employs a movement of a compressible fluid which discharges under pressure from the cylinder into a lower pressure area, and including compressors, hot air engines, steam enm'nes, and internal combustion engines, such as diesel engines. My filter can also be employed in the suction and discharge of a fan, a rotary compressor, jet engine, gas turbine, relief and safety valves, etc.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

What is claimed is:

1. An acoustic filter, comprising in combination: first substantially rigid wall means defining an annular chamber about a given axis; second substantially rigid wall means defining another chamber axially spaced in one direction from said first chamber; a conduit coupled to the inner side walls of said annular chamber and extending axially therefrom in an opposite direction; a filler material in said chambers, said filler material forming a plurality of small conduits defining the free volume of said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being predetermined according to the resonant frequency to be absorbed in accordance with the equation:

in which A equals the area of said ports, V equals said free volume, K equals a constant and f equals said resetnant frequency.

2. An acoustic filter, comprising, in combination: first metallic wall means defining an annular chamber about a given axis; second metallic wall means defining another chamber axially spaced in one direction from said first chamber; a conduit coupled to the inner sidewalls of said annular chamber and extending axially therefrom in an opposite direction; a filler material in said chambers; said filler material forming a plurality of small conduits defining the free volume of said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being pre-determined according to the resonant frequency to be absorbed in accordance with the equation:

AIM

in which A equals the area of said ports, V equals said free volume, K equals a constant and 1 equals said resonant frequency.

3. An acoustic filter, comprising, in combination: first substantially rigid wall means defining an annular chamber about a given axis; second substantially rigid wall means defining another chamber axially spaced in one direction from said first chamber; a conduit of truncated conical shape; said conduit having its larger end coupled to the inner sidewalls of said annular chamber and extending axially therefrom in an opposite direction; a filler material in said chambers; said filler material forming a plurality of small conduits defining the free volume of said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being pre-determined according to the resonant frequency to be absorbed in accordance with the equation:

in which A equals the area of said ports, V equals said free volume, k equals a constant and 1 equals said resonant frequency.

4. An acoustic filter, comprising, in combination: first substantially rigid Wall means defining an annular chamber about a given axis; second substantially rigid wall means defining another chamber axially spaced in one direction from said first chamber; means supporting said chambers in spaced apart relationship; a conduit coupled to the inner sidewalls of said annular chamber and extending axially therefrom in an opposite direction; a filler material in said chambers; said filler material forming a plurality of small conduits defining the free volume of said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being pre-determined according to the resonant frequency to be absorbed in accordance with the equation:

in which A equals the area of said ports, V equals said free volume, K equals a constant and 1 equals said resonant frequency.

5. An acoustic filter, according to claim 4, in which said means supporting said chambers in spaced apart relationship comprise a plurality of lug members rigidly interconnected between said chambers.

6. An acoustic filter, comprising in combination: first substantially rigid wall means defining an annular chamber about a given axis; second substantially rigid wall means defining another chamber, said another chamber being of pancake shape and axially spaced in one direction from said first chamber; a conduit coupled to the inner sidewalls of said annular chamber and extending axially therefrom in an opposite direction; a filler material in said chambers; said filler material forming a plurality of small conduits defining the free volume of said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being pre-determined according to the resonant frequency to be absorbed in accordance with the equation:

Alli Kf== in which A equals the area of said ports, V equals said free volume, K equals a constant and 1 equals said resonant frequency.

References Cited in the file of this patent UNITED STATES PATENTS 1,796,441 Compo Mar. 17, 1931 1,934,463 Hartsock Nov. 7, 1933 2,020,903 Nickelsen Nov. 12, 1935 2,037,884 Day Apr. 21, 1936 2,058,932 Wilson Oct. 27, 1936 2,164,365 Wilson July 4, 1939 2,323,955 Wilson July 13, 1943 2,869,671 Schlachter et al Jan. 20, 1959 FOREIGN PATENTS 664,331 France Apr. 22, 1929 275,495 Italy June 24, 1930 493,538 Great Britain Nov. 10, 1938 

